Nocardia spp. are ubiquitous soil organisms which may appear as primary opportunistic invaders in humans in cases of actinomycetoma or as secondary opportunistic pathogens in the immunocompromised host. In the former, the microorganisms become implanted in the subcutaneous tissues via trauma; whereas in the latter, either pulmonary, systemic or subcutaneous nocardiosis may occur. This infection is most often seen in patients suffering from cancer, chronic disorders, AIDS and those undergoing transplantation or immunosuppressive therapy. While the clinical picture and distribution of nocardiosis and actinomycetoma are different, their diagnosis is equally problematic due to the fact that it relies on the isolation and identification of the causative agent in culture. This often requires 2 to 3 weeks as Nocardiae are slow growers; in addition to an adequate sample which frequently calls for invasive procedures. Thus, in recent years, the need for alternative diagnostic tests has been emphasized. Even though several serological assays have been developed, they have yet to be as specific and/or sensitive as desired. This is mostly due to the lack of a systematic analysis, purification and characterization of Nocardia antigens. This proposal is aimed at the long term goal of defining and characterizing Nocardia antigens in order to be able to use them for the study of the host-parasite relationships in an actinomycetoma animal model and for the development of reliable serological tests. Specifically the research plan presents experiments aimed at the purification and characterization of a N. brasiliensis acid phosphatase and a 54 kDa molecule which has been recognized as a genus specific antigen.